Monday, September 11, 2017

nature | Lunar-origin studies are in flux. No current impact model stands out as
more compelling than the rest. Progress in several areas is needed to
rule out some theories, support others or direct us to new ones.

First, a better understanding of what happened between the formation
of the disk and the accumulation of the Moon from the disk is essential,
because this phase established the Moon's properties. Did mixing
homogenize the composition of the disk and the planet before the Moon
formed? Were volatile elements lost from the disk, and, if so, did the
pattern of loss vary with the disk's temperature? Canonical impacts
produce a mostly liquid disk whereas in the high-angular-momentum
impacts, the disks are initially largely vapour. Such disk-evolution
models are technically challenging and will require a multidisciplinary
approach incorporating both dynamics and chemistry.

Second,
the likelihood that a resonance altered the Earth–Moon angular momentum
needs to be assessed for a variety of physical states of the early
Earth and Moon and using state-of-the-art models for the tidal
interactions between them.

Finally, further
isotopic comparisons of lunar and terrestrial materials would be
extremely valuable. They should include highly refractory elements, such
as calcium, to test the equilibration model. Finding that an element
that could not have mixed in a vapour phase in 100 years is the same in
the Moon and Earth but different in Mars would argue against
equilibration; finding Earth–Moon isotopic differences in such a highly
refractory element would support it.

Oxygen
provides arguably the most important isotopic constraint on lunar
formation. The distinct oxygen isotopic compositions of the Earth–Moon
system, Mars and most meteorites reflect different initial compositional
reservoirs in the inner Solar System. This simplifies the
interpretation of oxygen compositions compared with elements such as
silicon, whose isotopic abundances are affected by later planet-forming
processes (such as crustal extraction). Increasing the precision of
oxygen isotope measurements could potentially rule out some impact
scenarios.

It remains troubling that all of
the current impact models invoke a process after the impact to
effectively erase a primary outcome of the event — either by changing
the disk's composition through mixing for the canonical impact, or by
changing Earth's spin rate for the high-angular-momentum narratives.

Sequences
of events do occur in nature, and yet we strive to avoid such
complexity in our models. We seek the simplest possible solution, as a
matter of scientific aesthetics and because simple solutions are often
more probable. As the number of steps increases, the likelihood of a
particular sequence decreases. Current impact models are more complex
and seem less probable than the original giant-impact concept.

A
clue may lie in Venus. The assumption that the Moon-forming impactor
had a composition very different from that of Earth is largely based on
what we know about Mars. We do not know the isotopic composition of
Venus, the planet most similar to Earth in both mass and distance from
the Sun. If Venus's composition proves similar to that of Earth and the
Moon, Mars would then seem to be an outlier, and an impactor composition
akin to Earth's would be more probable, removing many objections to the
canonical impact.

Determining the isotopic
composition of Venus's key elements will probably require a mission to
the planet. Such a tantalizing prospect reminds us how much there is
still to learn in our Solar System backyard.